Add tutorials for seismic wave

[core-man]

References

• https://migg-ntu.github.io/SeisTomo_Tutorials/seismology/intro/index.html
• https://migg-ntu.github.io/SeisTomo_Tutorials/seismology/seismic-wave/

Related to seismo-learn/maintenance#11

[core-man]

Some points we may want to further include after we finish the first version:

• Definition of the period, frequency, wavelength, wavenumber
• Emphasize the key point of wave propagation: a wave propagates the vibration
• Clarify the definition of phase and group velocity
• Clarify particle motion direction, phase direction, ray direction, slowness vector direction, wavenumber vector direction

Or if some of them are a little difficult for new babies, we may include them in the seismology repo.